This invention relates to apparatus and methods for determining when to replace a retaining ring used in substrate polishing operations (e.g., chemical-mechanical polishing).
Chemical mechanical polishing (CMP) is a process for planarizing the surface of a substrate (e.g., a semiconductor wafer). In a typical CMP process, a polishing surface of a polishing sheet (or pad) is covered with a slurry solution containing abrasive particles and one or more reactive chemicals. A substrate to be polished is held against the polishing surface by a carrier head in a recess defined by a substrate support surface and a retaining ring. The polishing surface and the carrier head are moved relative to one another causing the slurry to mechanically and chemically remove portions of the substrate surface.
The retaining ring serves to hold the substrate in position on the carrier head and improves the uniformity of the polishing process. During this process, however, the retaining ring is exposed to the polishing action of the slurry and, after a period of time, a significant portion of the retaining ring will have been worn away. After a certain amount of material has worn away, its ability to retain the substrate in place and its beneficial impact on polishing uniformity diminishes. Eventually, the retaining ring must be replaced to avoid detrimental impact on the quality and yield of the polishing process.
In one aspect, the invention features a retaining ring. The retaining ring has an inner surface exposed to contact a peripheral edge of a substrate to be polished against a polishing surface, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface.
In another aspect, the invention features a substrate polishing apparatus that includes a carrier head configured to hold a substrate against a polishing surface, and a retaining ring having an inner surface exposed to contact a peripheral edge of the substrate, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface.
Embodiments may include one or more of the following features.
The wear marker may comprise a visual indicator that is located at the outer surface of the polishing apparatus and is exposed for visual inspection while the substrate is being polished. The visual indicator may comprise a color change. The color change may result from a change in material composition between the bottom surface and the location of the visual indicator, or from a colorant applied to the outer surface. The wear marker may comprise a change in a structural feature of the outer surface. The structural feature change may comprise a hole extending from the outer surface to the inner surface; the hole preferably extends in a linear direction oriented at an acute angle relative to the bottom surface. Alternatively, the structural feature change may comprise a continuous groove that defines a plane that is substantially parallel to the bottom surface.
The wear marker may be exposed for detection at the bottom surface after the bottom surface has been worn away by a preselected amount. The wear marker and the bottom surface may be formed from different material compositions. The wear marker may be formed from a polymeric material, or a metal. The material compositions of the wear marker and the bottom surface may have different reflectivity characteristics. For example, in one embodiment, the wear marker is formed from a metal and the bottom surface is formed from a polymeric material.
The bottom surface may include a groove having a characteristic depth, and the wear marker may be exposed for detection after the depth of the groove has been reduced sufficiently by wearing. The wear marker may comprise a metallic surface disposed in the groove, an annular ring, or one or more spaced-apart wear marker plugs.
A detection system may be provided. The detection system may be configured to detect the wear marker and to generate a warning signal upon detection of the wear marker.
In another aspect, the invention features a substrate polishing method, in which one or more substrates are polishing against a polishing surface with a retaining ring having an inner surface exposed to contact a peripheral edge of the substrate, a bottom surface exposed to contact the polishing surface while the substrate is being polished, and a wear marker indicative of a preselected amount of wear of the bottom surface. At least a portion of the retaining apparatus is replaced when the bottom surface has been worn away by the preselected amount indicated by the wear marker.
The invention also features a substrate polishing method, in which one or more substrates are polished against a polishing surface with a substrate carrier that includes a substrate retaining ring with a wear marker indicative of a preselected amount of wear of the retaining ring, and a warning signal is generated upon detection of the wear marker.
The wear marker may be detected optically.
Among the advantages of the invention are the following. The invention enables CMP operators to determine when a retaining ring should be replaced based upon a simple visual inspection of the retaining ring before, during or after a CMP process. The invention also provides a system for automatically determining when a retaining ring should be replaced. The invention allows retaining rings to be efficiently used without risk of the detrimental impact on process quality and yield that might be caused by using overly worn retaining rings. The invention reduces processing costs by reducing materials costs (in the form of reducing premature disposal of retaining rings) and by reducing labor costs (in the form of reducing CMP operator time required to monitor retaining ring life).
Other features and advantages will become apparent from the following description, including the drawings and the claims.